Long term dimensional stability is required for support structures in many instruments having optical components focussed on distant objects. For example, imaging systems on future space flights such as the Saturn-bound Cassini spacecraft impose very strict requirements on the metering rods in the camera athermalizing system. The metering rods move optical elements relative to the camera to compensate for temperature variation in the system. The next generation systems represented by the camera to be used on the Cassini spacecraft use an imaging design having higher performance goals and which are very sensitive to dimensional errors. The metering rods must satisfy requirements for very low thermal expansivity and temporal stability more rigorous than ever required before.
Not only must the material meet the dimensional stability requirements, it must also be machineable and have mechanical strength required for its use. Alloys of iron and nickel such as Super Invar (Fe-Co-Ni) and INVAR 36 (Fe-36Ni) are known to have remarkably low coefficients of thermal expansion (CTE) near room temperature. This effect is believed due to the magnetic properties of the family of alloys. Though Super Invar has superb dimensional stability at room temperature, it is not suitable for use as supports in precision instruments due to its highly composition-dependent, irreversible phase transformation and temperature dependent temporal stability. It is also very difficult to fabricate.
INVAR 36 has more practical applications since it is easier to fabricate and has low CTE over a wide range of temperatures. The CTE of INVAR 36 has been reported to vary from -0.6 to +3.00 ppm/.degree.C. in the temperature range of -70.degree. to +100.degree. C. With careful controls, it is commercially practical to produce INVAR 36 with a narrower range of CTE values, e.g. 0.8 to 1.6 ppm/.degree.C. in the range of 30.degree. to 100.degree. C. However, the excellent thermal stability of INVAR 36 is not accompanied by isothermal temporal (long term) stability. Temporal instability values as high as +11.0 ppm/day at temperatures of 20.degree. to 70.degree. C. have been reported for Invars of varying composition and subjected to varying thermomechanical treatments.
Prior studies conducted on commercial Invar alloys indicate that impurities have a pronounced effect on the coefficient of thermal expansion. The thermal expansion is also affected by thermal and mechanical treatments. Temporal stability is known to be affected by test temperature, temperature changes, heat treatment and forming operations. The temporal stability decreases from room temperature to 60.degree. C. in prior INVAR 36 materials. Heat treatment which is intended to stabilize and/or stress relieve INVAR 36 and accelerate the aging process may increase or decrease temporal stability. Studies show that commercial INVAR 36 expands with rates varying from 1.5 to 27 ppm/year.
However, conventional Invar alloy production does not have the necessary controls to produce high purity materials and, in fact, results in the introduction of impurities during alloying operations.
In conventional Invar alloy production, such as air or vacuum melting, antioxidants are added during the melting operation and refractory materials such as carbon-graphite can be introduced into the melt from the furnace lining during melting or from the lining of the ingot mold when the melt is poured. There is limited chemistry control of commercially produced ingots; chemical composition is only analyzed during the melt stage. A sample analysis is made during the melting stage. Adjustments to the alloy composition are made based on the analysis. There is no final analysis. The last analysis which is reported as the alloy composition may not represent the actual composition throughout the melt.